"As geology plainly proclaims that each land has undergone great physical 

 changes, we might have expected to find that organic beings have varied under 

 nature, in the same way as they have varied under domestication. And if there 

 has been any variability under nature, it would be an unaccountable fact if natural 

 selection had not come into play. It has often been asserted, but the assertion is 

 incapable of proof, that the amount of variation under nature is a strictly limited 

 quantity. Man, though acting on external characters alone and often capriciously, 

 can produce within a short period a great result by adding up mere individual 

 differences in his domestic productions, and everyone admits that species present 

 individual differences. But, besides such differences, all naturalists admit that 

 natural varieties exist, which are considered sufficiently distinct to be worthy of 

 record in systematic works. No one has drawn any clear distinction between 

 individual differences and slight varieties ; or between more plainly marked 

 varieties and subspecies, and species. On separate continents, and on different 

 parts of the same continent when divided by barriers of any kind, and on outlying 

 islands, what a multitude of forms exist, which some experienced naturalists rank 

 as varieties, others as geographical races or subspecies, and others as distinct, 

 though closely allied species ! 



" If then, animals and plants do vary, let it be ever so slightly or slowly, why 

 should not variations or individual differences, which are in any way beneficial, be 

 preserved and accumulated through natural selection, or the survival of the fittest ? 

 If man can by patience select variations useful to him, why, under changing and 

 complex conditions of life, should not varieties useful to nature's living products 

 often arise, and be preserved or selected ? What limit can be put to this power, 

 acting during long ages and rigidly scrutinising the whole constitution, structure, 

 and habits of each creature, — favouring the good and rejecting the bad? " 



Darwin, The Origin of Species. 



